The specific aims of the proposed study are to determine the contribution of arterial blood flow and of venous tone to the velocity of venous blood flow and the effect of local temperature and heparin on venous flow velocity and venous tone. Arterial blood flow and venous volume-pressure curves will be measured by venous occlusion plethysmography; velocity of venous blood flow will be determined by the rate of travel of a radioisotope from a foot vein to the femoral vein. Humoral substances that affect only arterial flow or only venous tone will be administered to determine the effects on venous flow velocity. The effects of local heat and cold and of intravenous heparin on arterial flow, venous tone, and venous flow velocity will also be studied. Correlation coefficients will be calculated to determine the importance of each variable. Vasodilator drugs which are likely to increase venous flow velocity will be studied for their effect on arterial flow, venous tone, and venous flow velocity. A slowing of venous flow velocity with venous stasis is probably one of the most important factors leading to venous thrombosis; a continual speeding of venous flow by a vasodilator drug may be determined to be worthy of a large scale clinical trial.